Reactor and process for preparing phosgene

ABSTRACT

A reactor ( 1 ) for preparing phosgene by gas-phase reaction of carbon monoxide and chlorine in the presence of a solid catalyst, which reactor has a bundle of parallel catalyst tubes ( 2 ) aligned in the longitudinal direction of the reactor whose ends are fixed in tube plates ( 3 ) and have a cap ( 4 ) at each end of the reactor ( 1 ), and has deflection plates ( 6 ) which are aligned perpendicular to the longitudinal direction of the reactor in the intermediate space ( 5 ) between the catalyst tubes ( 2 ) and leave free passages ( 7 ) located alternately opposite one another on the interior wall of the reactor, and in which the catalyst tubes ( 2 ) are charged with the solid catalyst, the gaseous reaction mixture is passed from one end of the reactor via one cap ( 4 ) through the catalyst tubes ( 2 ) and is taken off from the opposite end of the reactor ( 1 ) via the second cap ( 4 ) and a liquid heat transfer medium is passed through the intermediate space ( 5 ) around the catalyst tubes ( 2 ), where the reactor ( 1 ) has no tubes in the region of the passages ( 7 ), is proposed.

The invention relates to a reactor for preparing phosgene by gas-phasereaction of carbon monoxide and chlorine in the presence of a solidcatalyst, to an apparatus and to a process for preparing phosgene usingthe reactor or the apparatus.

Phosgene is prepared industrially in a catalytic gas-phase reaction ofcarbon monoxide and chlorine in the presence of a solid catalyst,preferably activated carbon. The reaction is strongly exothermic: theenthalpy of formation is −107.6 kJ/mol. The reaction is generallycarried out in a shell-and-tube reactor by the process described inUllmanns Enzyklopädie der technischen Chemie, Vol. A 19, pages 413 to414. In this process, the granular catalyst, which has a particle sizein the range from 3 to 5 mm, is installed in tubes having an internaldiameter of from 50 to 70 mm. The reaction commences at from 40 to 50°C., the temperature in the tubes rises to about 400° C. and then quicklydrops. Carbon monoxide is used in a small excess to ensure that all thechlorine is reacted and chlorine-free phosgene is obtained. The reactioncan be carried out at atmospheric pressure or under superatmosphericpressure, frequently at from 2 to 3 bar, so that the phosgene can becondensed by means of cooling water.

Phosgene is an important chemical in the preparation of intermediatesand end products in virtually all branches of chemistry. The largestapplication in terms of quantity is the preparation of diisocyanates forpolyurethane chemistry, in particular tolylene diisocyanate anddiphenylmethane 4,4-diisocyanate.

To improve the removal of the heat of reaction by means of the heattransfer medium circulating between the catalyst tubes, deflectionplates are installed between the catalyst tubes to generate transverseflow of the heat transfer medium over the catalyst tubes.

Known shell-and-tube reactors for the preparation of phosgene arecompletely filled with tubes to obtain maximum utilization of space inthe interior of the reactor and have deflection plates between thecatalyst tubes. These plates are kept relatively short, i.e. they areinstalled in a deflection region which does not extend as far as theinterior wall of the reactor but instead they leave a proportion of fromabout 25 to 30% of the total reactor cross section free, in order tolimit the pressure drop experienced by the heat transfer medium and thusthe operating costs for the circulation pump for the heat transfermedium. In the deflection region, the flow profile of the heat transfermedium is altered around the catalyst tubes from transverse flow tolongitudinal flow. The catalyst tubes are not cooled as well, so thatcorrosion problems occur on the catalyst tubes in the deflection region.

It is an object of the present invention to avoid corrosion problems onthe catalyst tubes in the deflection region and to provide a reactor forpreparing phosgene which makes possible an increased specificsuperficial velocity and thus has a higher capacity.

We have found that this object is achieved by a reactor for preparingphosgene by gas-phase reaction of carbon monoxide and chlorine in thepresence of a solid catalyst, which reactor has a bundle of parallelcatalyst tubes aligned in the longitudinal direction of the reactorwhose ends are fixed in tube plates and have a cap at each end of thereactor, and has deflection plates which are aligned perpendicular tothe longitudinal direction of the reactor in the intermediate spacebetween the catalyst tubes and leave free passages located alternatelyopposite one another on the interior wall of the reactor, and in whichthe catalyst tubes are charged with the solid catalyst, the gaseousreaction mixture is passed from one end of the reactor via one capthrough the catalyst tubes and is taken off from the opposite end of thereactor via the second cap and a liquid heat transfer medium is passedthrough the intermediate space around the catalyst tubes, the reactor ofthe present invention having no tubes in the region of the passages.

In the present context, the term passage refers to the region betweenthe free end of a deflection plate and the interior wall of the reactor.

It was found that leaving the interior of the reactor free in the regionof the passages as specified according to the present invention enablesthe capacity of a reactor for preparing phosgene to be increased by afactor of from 1.5 to 2.0 compared to a reactor completely filled withtubes at an unchanged interior volume and an increased amount ofcoolant, although a smaller total number of catalyst tubes isaccommodated in the reactor.

In addition, it has been found that the reactor of the present inventionfor preparing phosgene by gas-phase reaction of carbon monoxide andchlorine in the presence of a solid catalyst can be constructed evenwithout compensators for evening out thermal stresses in the reactorwall: it has been found that the temperature increase of the catalysttube walls caused by the exothermic reaction occurs only in subregionsof the tube walls, and can therefore be largely accommodated over thelength of the catalyst tubes and only produces small stresses in thewelded seams of the catalyst tube joins at the tube plates. This effectincreases with increasing length of the catalyst tubes, especially whenthe length of the catalyst tubes is greater than 2.5 m. As a result ofcompensators on the reactor wall being omitted, the reactor becomesstiffer overall and the tube plates can therefore be made less thick.This advantageously leads to lighter apparatuses and, in addition, foran unchanged total length of the apparatus, the length of the catalysttubes which can be utilized for accommodating the catalyst chargebecomes greater, accompanied by a corresponding increase in the runningtime of the reactor. Thus, for example, it is possible to achieve alengthening of the catalyst charge by about 40 cm, resulting in arunning time increase for the reactor of the order of one year. Inaddition, the reactor without compensator is cheaper.

The geometry of the reactor of the present invention is in principle notsubject to any restrictions. It preferably has a cylindrical shape,although other shapes having, for example, a square or rectangular crosssection are also possible.

A bundle, i.e. a large number, of catalyst tubes are located in thereactor parallel to one another in the longitudinal direction of thereactor. The number of catalyst tubes is preferably in the range from100 to 10000, in particular from 1000 to 3500.

The catalyst tubes are made of a corrosion-resistant material, forexample stainless steel, preferably duplex steel 1.4462, stainless steel1.4571 or stainless steel 1.4541. The entire reactor is preferably madeof the materials mentioned above, especially of duplex or stainlesssteel.

Each catalyst tube preferably has a wall thickness in the range from 2.0to 4.0 mm, in particular from 2.5 to 3.0 mm, and an internal tubediameter in the range from 20 to 90 mm, preferably in the range from 30to 35 mm.

The catalyst tubes preferably have a length in the range from 1.5 to 6.0m, in particular in the range from 2.0 to 3.5 m.

The catalyst tubes are preferably arranged in the interior of thereactor in such a way that the ratio of the spacing of the centers ofdirectly adjacent catalyst tubes to the external diameter of thecatalyst tubes is in the range from 1.15 to 1.4, preferably in the rangefrom 1.2 to 1.3, and that the catalyst tubes are arranged in triangularpitch in the reactor.

At the two ends, the contact tubes are fixed in a liquid-tight manner,preferably welded, into tube plates. The tube plates likewise consist ofa corrosion-resistant material, preferably stainless steel, inparticular duplex steel, particularly preferably the same material asthe catalyst tubes.

The internal diameter of the reactor is, if it is a cylindricalapparatus, from 0.5 to 6.0 m, preferably from 1.0 to 3.0 m.

Both ends of the reactor are closed off from the outside by caps. Thereaction mixture is fed into the catalyst tubes through one cap, and theproduct stream is taken off through the cap at the other end of thereactor.

Gas distributors for uniformly distributing the gas stream, for examplein the form of a plate, in particular a perforated plate, are preferablylocated in the caps.

In the intermediate space between the catalyst tubes, deflection platesare arranged perpendicular to the longitudinal direction of the reactorso as to leave free passages located alternately opposite one another onthe interior wall of the reactor. The deflection plates deflect the heattransfer medium circulating in the interior of the reactor in theintermediate space between the catalyst tubes in such a way that theheat transfer medium flows transversely over the catalyst tubes, thusimproving heat removal. To achieve this advantageous transverse flowover the catalyst tubes, the deflection plates have to leave passagesfree for the heat transfer medium alternately at the opposite sides ofthe interior wall of the reactor.

The number of deflection plates is preferably from about 6 to 21. Theyare preferably arranged at equal distances from one another, but thelowermost deflection plate and the uppermost deflection plate are eachparticularly preferably at a greater distance from the tube plate thanthe distance between two successive deflection plates, preferably by afactor of about 1.5.

The passages left free can in principle have any shape. In the case of acylindrical reactor, they preferably have the shape of a segment of acircle.

It is preferred that all deflection plates leave equal-sized passagesfree.

The area of each passage is preferably from 5 to 20%, in particular from8 to 14%, of the reactor cross section.

It is preferred that the deflection plates do not form a seal around thecatalyst tubes and allow a leakage flow of up to 40% by volume of thetotal flow of the heat transfer medium.

For this purpose, gaps having a width of from 0.1 to 0.6 mm, preferablyfrom 0.2 to 0.3 mm, are provided between the catalyst tubes and thedeflection plates.

It is advantageous for the deflection plates to form a liquid-tight sealagainst the interior wall of the reactor except in the regions of thepassages, so that no additional leakage flow occurs at the interior wallof the reactor.

The deflection plates are made of a corrosion-resistant material,preferably stainless steel, in particular duplex steel, and preferablyhave a thickness of from 8 to 30 mm, preferably from 10 to 20 min.

The catalyst tubes are charged with a solid catalyst, preferablyactivated carbon. The catalyst bed in the catalyst tubes preferably hasa free volume of from 0.33 to 0.5, in particular from 0.33 to 0.40.

Both tube plates are preferably provided with vent and/or drainageholes, in particular at a plurality, preferably from 2 to 4, placesdistributed symmetrically over the reactor cross section, whose openingstoward the outside preferably lead into half-shells welded onto theexterior wall of the reactor.

To balance thermal expansions, a compensator is advantageously providedin the outer wall of the reactor.

The introduction and discharge of the heat transfer medium into and fromthe intermediate space between the catalyst tubes preferably occurs viaports or part-ring channels on the outer wall of the reactor which haveopenings into the interior of the reactor and preferably have a circularor rectangular cross section and an opening ratio in the range from 5 to50%, preferably from 15 to 30%.

The reactor is preferably symmetrical about a cross-sectional plane inthe middle of the reactor. In this preferred embodiment, an uprightreactor thus has upper and lower parts which are identical. For thepurposes of the present invention, this means that all connections andthe reactor brackets which serve to support the reactor are arrangedsymmetrically. The catalyst is consumed to a different extent indifferent regions as the reaction progresses as a result of themigration of the hot spot zone. Similarly, the catalyst tubes aresubject to different stresses in different regions, with the greateststress occurring in the region of the hot spot zone. In this hot spotzone, there is firstly corrosion of the interior wall of the catalysttubes leading to the danger of the catalyst tubes leaking. In the caseof leaking tubes, the entire reactor has to be emptied of catalystcharge and heat transfer medium and the catalyst charge which has beentaken out has to be flushed with nitrogen for a number of days. Theleaking tube or tubes have to be replaced and refilled with catalyst.This danger can be alleviated by means of the above-describedsymmetrical configuration which makes it possible to turn the reactor ingood time before a certain critical degree of corrosion is reached, sothat the hot spot region moves to a part of the catalyst tubes which haspreviously been subject to less stress. This enables the operating timeof the reactor to be increased considerably, frequently doubled.

In an advantageous embodiment, the reactor of the present invention hasa plurality of zones, in particular two or three zones, with differentcooling being applied in the zones. Particular preference is given to atwo-zone reactor. This embodiment can be used particularlyadvantageously when chlorine which has largely been freed of bromine isused as starting material for the reaction to form phosgene. It has inthis case been observed that the reaction to form phosgene proceeds moreslowly than when bromine-contaminated chlorine is used. The reason forthis could be free-radical formation of bromine with activated carbon,which accelerates the reaction with chlorine. If the phosgene formationreaction proceeds more slowly, for example because of the use ofchlorine which has largely been freed of bromine, it would in principlebe possible to increase the total conversion to phosgene by increasingthe inlet temperature of the heat transfer medium. However, this ispossible only to a limited extent, since the liquid heat transfer mediumcan boil at the outer wall of the catalyst tubes and consistent heattransfer and consistent reaction conditions are therefore no longerensured.

For this reason, an advantageous embodiment as two-zone reactor withdifferent cooling of the two zones, namely with stronger cooling in thefirst zone in the flow direction of the reaction mixture, i.e. the mainreaction region, and with weaker cooling in the second zone, i.e. in theafter-reaction region or in the region in which residual conversionoccurs, is provided. Cooling in the first zone is preferably carried outusing a colder heat transfer medium than in the second zone.

The two zones are separated from one another in a liquid-tight manner bymeans of a dividing plate in the intermediate space between the catalysttubes so that the heat transfer medium cannot flow from one zone intothe other within the reactor. The catalyst tubes are rolled into theseparating plates or hydraulically widened so as to form a seal. Thedividing plate advantageously has a thickness in the range from 15 to 60mm, preferably from 30 to 50 mm.

In the case of three-zone or multizone reactors, two or more dividingplates are correspondingly provided to separate the individual zonesfrom one another.

To even out thermal stresses, a compensator is advantageously providedon the reactor wall in each reaction zone.

In the case of a two-zone reactor, the length ratio of zone 1 to zone 2can be in the range from 1:1 to 3:1, advantageously 2:1.

The total length of the catalyst tubes of a two-zone reactor isfrequently in the range from 2.5 to 6.0 m, preferably in the range from3.0 to 4.0 m.

The progress of the reaction can be monitored by means of the exittemperature of the heat transfer medium from the second zone. If thetemperature there increases appreciably, it indicates that the reactionfront is migrating into the lower region of the reactor.

A smaller volume flow of heat transfer medium is required in the secondzone compared to the first zone. The heat transfer medium stream leavingthe second zone can be combined with the heat transfer medium streamentering the first zone, so that a greater total volume of heat transfermedium flows through the first zone.

The deflection plates for the heat transfer medium preferably have notubes in all zones of the two-zone, three-zone or multizone reactor.

A sheath for accommodating a multithermocouple with two or moretemperature measuring points, whose open end is below the reactor, ispreferably provided in at least one of the catalyst tubes. The sheath isinserted via a port through the lower cap of the reactor. Positioning itin the lower catalyst tube region is advantageous because of the lowerreaction temperature in this region. In general, the temperature in theupper catalyst tube region of a phosgene reactor is higher, with acorrespondingly greater stress on the materials as a result of theaggressive reaction mixture comprising phosgene and chlorine. If, on theother hand, the sheath is located in the lower catalyst tube region asproposed, the stress on the material of the sheath is lower because ofthe lower temperatures prevailing there. The multithermocouple ispreferably hard-wired and has two or more, preferably up to 10,preferably regularly spaced temperature measuring points. Thetemperature measurement allows the progress of the reaction to bemonitored and measurements which allow conclusions to be drawn as to thecatalyst activity and the appropriate time of replacement of thecatalyst can be obtained.

The invention also provides an apparatus for preparing phosgene bygas-phase reaction of carbon monoxide and chlorine in the presence of asolid catalyst, which comprises two or more reactors of theabove-described type which are connected in series, wherein aconcentration measuring point for the residual chlorine content and/or atemperature measuring point is/are preferably provided in the connectingpiece between the lower cap of the upper reactor and the upper cap ofthe lower reactor.

Preference is given to two reactors being connected in series and thesecond reactor having catalyst tubes having a larger internal tubediameter than in the first reactor, in particular an internal tubediameter in the range from 20 to 110 mm, preferably in the range from 60to 90 mm.

In a further embodiment, it is possible to connect two or more reactorsdirectly in series without caps located in between. Preference is givento connecting two reactors directly in series. The adjacent tube plates,i.e. the lower tube plate of the first reactor and the upper tube plateof the second reactor, are advantageously joined to one another by meansof spacers which keep them some distance apart so as to allow transversemixing of the reaction mixture leaving the catalyst tubes of the firstreactor before it enters the second reactor.

A reactor as described above or an apparatus can advantageously beenclosed by a safety chamber. In this case, the second and/or furtherreactors preferably have a smaller external dimension than the upstreamreactor, so that these can be replaced more easily.

The invention also provides a process for preparing phosgene bygas-phase reaction of carbon monoxide and chlorine in the presence of asolid catalyst in a reactor as described above.

The liquid heat transfer medium used in the process of the presentinvention is preferably water, aqueous sodium hydroxide solution or oneor more, preferably chlorinated hydrocarbons, in particularmonochlorobenzene.

The process of the present invention is not restricted in respect of theflow directions of gaseous reaction mixture and heat transfer medium; itis equally possible to pass the gaseous reaction mixture and the liquidheat transfer medium through the reactor in cross-countercurrent or incross-cocurrent. The gaseous reaction mixture can equally well be passedvia the caps through the catalyst tubes of the reactor from the top orfrom the bottom.

The catalyst installed in the catalyst tubes is preferably activatedcarbon, in particular in the form of spheres, cones, cylinders,extrudates, rings or pellets. The region of the catalyst tubes nearestthe end at which the gaseous reaction mixture is fed in is particularlypreferably filled with an inert material to a length of from 5 to 20%,preferably a length of from 5 to 10%, of the total tube length of thecatalyst tubes.

An at least partially open-pored carbon foam can advantageously beinstalled as catalyst in the catalyst tubes. Such catalysts areparticularly advantageous because of the very high internal surfacearea.

The catalyst tubes are preferably configured, in particular by means ofthe arrangement in the interior of the reactor, in such a way that theirheat transfer coefficient to the heat transfer medium is in the rangefrom 500 to 2000 W/m²/K, in particular in the range from 1000 to 1500W/m²/K.

The process of the present invention is preferably carried out bypassing carbon monoxide and chlorine in a molar ratio in the range from1.01 to 1.10, in particular in the range from 1.03 to 1.06, through thecatalyst tubes from the top or from the bottom, with the longitudinalaxis of the reactor being aligned vertically.

In the process of the present invention, the pressure in the reactor ispreferably from 2 to 10 bar absolute, in particular from 3 to 5 barabsolute.

The superficial velocity through the reactor is preferably from 0.5 to 4kg of phosgene per second per m² of reaction tube cross-sectional areathrough which flow occurs, in particular from 1.5 to 3 kg of phosgeneper second per m² of reaction tube cross-sectional area through whichflow occurs. The per superficial velocity is thus increased compared toconventional phosgene reactors.

The invention is illustrated below with the aid of a drawing andexamples.

In the drawing:

FIG. 1 shows a longitudinal section of a first preferred embodiment of areactor according to the present invention with cross-countercurrentflow of reaction mixture and heat transfer medium,

FIG. 2 shows a longitudinal section of a further preferred embodiment ofa reactor according to the present invention in which reaction mixtureand heat transfer medium are conveyed in cross-cocurrent and thecatalyst tubes are filled with an inert material in the region where thereaction mixture enters,

FIG. 3 shows a cross section in the plane A-A through a reactoraccording to the present invention shown in FIG. 1 or FIG. 2,

FIG. 4 shows part of the cross section of FIG. 3,

FIGS. 5 to 7 each show parts of the longitudinal sections in FIG. 1 orFIG. 2,

FIG. 8 shows a preferred embodiment of a two-zone reactor,

FIG. 9 shows a detail of the reactor of FIG. 8,

FIG. 10 shows a preferred embodiment of a three-zone reactor,

FIG. 11 shows a preferred embodiment of two reactors located directlyabove one another, with both reactors having no tubes in the deflectionregion of the heat transfer medium,

FIG. 12 shows a further preferred embodiment of two reactors arrangeddirectly above one another, with the second reactor having tubes in allplaces,

FIG. 13 shows an apparatus having two reactors connected in series and

FIG. 14 shows a section from FIG. 13 with a sheath for accommodating athermocouple.

In the figures, identical reference numerals denote identical oranalogous features.

FIG. 1 shows a longitudinal section of a preferred embodiment of areactor 1 according to the present invention having a bundle of catalysttubes 2 which are fixed parallel to one another in the longitudinaldirection of the reactor 1 and are sealed into tube plates 3, with caps4 at both ends of the reactor 1 and gas distributors 12 preferablylocated therein. In the intermediate space 5 between the catalyst tubes2, through which the liquid heat transfer medium flows, deflectionplates 6 are arranged perpendicular to the longitudinal direction of thereactor so as to leave free passages 7 located alternately opposite oneanother on the interior wall of the reactor.

Ports or segments of circular channels 11 are provided for introductionand discharge of the heat transfer medium. To even out thermal stresses,a compensator 10 is provided on the reactor wall.

The further preferred embodiment shown in FIG. 2 differs from theembodiment shown in FIG. 1 in the flow of reaction mixture and heattransfer medium (cross-cocurrent).

The catalyst tubes 2 are preferably, as shown in FIG. 2, filled with aninert material in the region where the gaseous reaction mixture enters.

The cross-sectional view in FIG. 3 shows the preferred shape, namelythat of a segment of a circle, of the passages 7 left free by thedeflection plates 6 in the region of the interior wall of the reactor.

FIG. 4 shows the preferred arrangement of the catalyst tubes 2 intriangular pitch, i.e. with the same distance t between the centers ofdirectly adjacent catalyst tubes.

The enlarged view in FIG. 5 shows the preferred provision of gaps 8between catalyst tubes 2 and deflection plates 6.

FIG. 6 shows the preferred arrangement of vent and/or drainage holes 9to the outside in the tube plates 3. The opening of the vent and/ordrainage holes to the outside is preferably, as shown in FIG. 6, coveredby a welded-on half shell as collector.

FIG. 7 shows another type of venting, by means of venting ports 13. Forthis purpose, a hole is drilled into the reactor wall and a port iswelded on, preferably about 20 mm below the tube plate.

The reactor shown in longitudinal section in FIG. 8 has a dividing plate14 which divides the intermediate space 5 between the catalyst tubes 2into a first, upper zone and a second, lower zone in a liquid-tightmanner. It can be seen from the figure that, in a preferred way ofcarrying out the process, a substream of the heat transfer mediumleaving the second zone can be mixed into the stream of heat transfermedium fed into the first zone.

FIG. 9 shows a detail of the reactor shown in FIG. 8, namely theliquid-tight rolling-in or hydraulic tube widening of the catalyst tube2 into the dividing plate 14, thus closing the gap 15 between catalysttube 2 and dividing plate 14.

FIG. 10 shows a further preferred embodiment of a reactor 1 having threezones separated from one another in a liquid-tight manner by dividingplates 14.

FIG. 11 shows an arrangement of two reactors connected directly inseries, with the lower cap of the upper reactor and the upper cap of thelower reactor being omitted. A spacer 16 is provided between the lowertube plate of the upper reactor and the upper tube plate of the lowerreactor. Both reactors have no tubes in the deflection region for theheat transfer medium.

The embodiment shown in FIG. 12 differs from the previous embodiment inthat the second reactor has tubes in all places, i.e. it is providedwith catalyst tubes even in the deflection region for the heat transfermedium.

FIG. 13 shows an apparatus comprising two reactors 1 connected in seriesand having a sheath 17 which opens to the outside via the lower cap 4 ofthe first reactor 1 and serves to accommodate a multithermocouple 18. Inthe connecting piece between the two reactors 1, a concentrationmeasuring cell 19 for the residual chlorine content and a temperaturemeasuring point 20 are provided.

The section in FIG. 14 shows the arrangement of the multithermocouple 18with a plurality of temperature measuring points in the sheath 17 whichopens to the outside via the port 22 in the lower cap 4 of the reactor1. The detail depicted in FIG. 14 also shows the support 21 for thecatalyst in the catalyst tube 2.

The invention is illustrated below with the aid of an example:

7087 kg/h of chlorine and 2913 kg/h of carbon monoxide were passedthrough the catalyst tubes of a cylindrical shell-and-tube reactorhaving 1256 catalyst tubes which each had a tube length of 3000 mm, aninternal tube diameter of 39.3 mm and a wall thickness of 2.6 mm andwere filled to a length of 200 mm with an inert ceramic material in theregion where the reaction mixture was fed in and subsequently filled toa length of 2700 mm with a bed of a cylindrical catalyst comprisingactivated carbon and having a diameter of the cylindrical catalystparticles of 4 mm and a length of 5 mm and a porosity of 0.404 at anadmission pressure of the reaction gas, i.e. a pressure of the reactiongas upstream of the reactor, of 4 bar absolute to give a product streamof about 10000 kg/h of phosgene.

The reactor was provided with ten deflection plates which left freepassages having the shape of a segment of a circle and eachcorresponding to about 15% of the total reactor cross section which werelocated alternately in the region between the free end of eachdeflection plate and the interior wall of the reactor.

The catalyst tubes were made of duplex steel 1.4462.

A reaction mixture of carbon monoxide and chlorine in a molar ratio of1.04226 was passed from the top downward through the catalyst tubes. Theliquid cooling medium, viz. monochlorobenzene, was passed through thereactor from the bottom upward in the region between the catalyst tubes.

The inlet temperature of the reaction mixture was 50° C. The inlettemperature of the liquid cooling medium, viz. monochlorobenzene, was60° C.

The abovementioned apparatus and process conditions were left unalteredfor a comparative experiment according to the prior art and for anexperiment according to the process of the invention. The onlydifference was that the reactor according to the prior art had a fullcomplement of tubes in the region of the passages for the heat transfermedium, i.e. about 200 tubes were present in the region of the passagesat the ends of the deflection plates.

In contrast, the reactor according to the present invention was leftfree of tubes in the deflection region for the heat transfer medium,i.e. in the region of the passages at the ends of the deflection plates.

For the reactor according to the prior art, the following table showsthe significant differences between catalyst tubes in the deflectionregion for the heat transfer medium and catalyst tubes over which heattransfer medium flows transversely, i.e. catalyst tubes outside thedeflection region for the heat transfer medium:

Catalyst tubes over Catalyst tubes in the which transverse flowdeflection region occurs Heat transfer coefficient 200 W/m²/K 800 W/m²/Kfrom cooling medium to tube wall Tube wall temperatures 295.9° C. 151.8°C. on the inside Mean maximum reaction 568.5° C. 545.5° C. temperaturein the catalyst bed

The heat transfer coefficient from the cooling medium to the tube wallof 200 W/m²/K for the catalyst tubes in the deflection region wassignificantly lower than the heat transfer coefficient of 800 W/m²/K forthe catalyst tubes over which transverse flow occurs.

This result can be explained by predominantly longitudinal flow of theheat transfer medium but virtually no transverse flow occurring in thedeflection region filled with tubes at the interior wall of the reactoraccording to the prior art. Accordingly, the temperature of the tubesrose to 295.9° C. which damages the material. The heat transfer mediumeven began to boil in places. As a result of the relatively large gasbubbles, cooling deteriorated. Decomposition of the heat transfer mediumformed deposits on the outside of the catalyst tubes, and these depositshad an insulating effect.

Severe material corrosion of about 0.8 mm per year on the catalyst tubesin the deflection region and some carbonization of the heat transfermedium monochlorobenzene on the cooling medium side of the catalysttubes were thus found in a long-term test using a reactor according tothe prior art.

In comparison, the heat transfer coefficient from the heat transfermedium to the tube wall was increased by a factor of four in the regionswith transverse flow. As a consequence, the wall temperature reached onthe inside of the catalyst tubes was 151.8° C. which is not damaging tothe tube material and is still below the boiling point of the heattransfer medium monochlorobenzene under the pressure conditions in theheat transfer medium around the catalyst tubes, viz. 2.5 bar absolute.As a result, no damage to the catalyst tubes was found in the transverseflow region.

Furthermore, the maximum rise in the mean reaction temperature in thecatalyst bed in catalyst tubes over which transverse flow occurred was545.5° C. and thus significantly lower than the 568.5° C. in thecatalyst tubes in the deflection region.

On the other hand, in the reactor according to the present invention,the catalyst tubes were omitted in the region of the passages at theends of the deflection plates, i.e. in the deflection region for theheat transfer medium. The reactor according to the present inventiontherefore had only catalyst tubes over which the heat transfer mediumflowed transversely, with the above-described significant processengineering advantages over the catalyst tubes over which predominantlylongitudinal flow occurs in the deflection region for the heat transfermedium.

The method employed in the process of the present invention enabledconsiderable improvements in economics to be achieved: starting out fromthe same reactor but, as a difference from the prior art, without tubesin the deflection regions for the heat transfer medium, as illustratedin the example, the throughput of the gaseous reaction mixture was ableto be approximately doubled when the amount of cooling medium wassimultaneously increased compared to a reactor according to the priorart, resulting in a corresponding increase in the capacity of thereactor and a lengthening of the period of operation of the reactor.

1-28. (canceled)
 29. A process for preparing phosgene by gas-phasereaction of carbon monoxide and chlorine in the presence of a solidcatalyst in one or more cylindrical reactors which has/have a bundle ofparallel catalyst tubes which are aligned in the longitudinal directionof the reactor and whose ends are fixed in tube plates which havedischarge holes therein, with a cap at each end of the reactor anddeflection plates which are aligned perpendicular to the longitudinaldirection of the reactor in the intermediate space between the catalysttubes and leave free passages located alternately opposite one anotheron the interior wall of the reactor, and a compensator positioned in thereactor wall and ports or part-ring channels provided on the reactorwall for the introduction and discharge of the heat transfer medium andthe openings of these ports or part-ring channels to the interior of thereactor have a circular or rectangular cross-section and an openingratio in the range from 5 to 50%, the process comprising: charging thecatalyst tubes with the solid catalyst, passing a gaseous reactionmixture comprising the carbon monoxide and the chlorine from one end ofthe reactor via one cap through the catalyst tubes, discharging thegaseous reaction mixture from the opposite end of the reactor via thesecond cap, and passing a liquid heat transfer medium through theintermediate space around the catalyst tubes, wherein the reactor (thereactors) for carrying out the process has/have no tubes in the regionof the passages, wherein gaps ranging from 0.1 to 0.6 mm are presentbetween the catalyst tubes and the deflection plates are fixed in aliquid-tight manner to the interior wall of the reactor with theexception of the regions of the passages.
 30. The process as claimed inclaim 29, wherein the deflection plates are configured as segments of acircle, all deflection plates each leave equal-sized passages free andthe area of each passage ranges from 5 to 20% of the cross section ofthe reactor.
 31. The process as claimed in claim 29, wherein from 100 to10,000 catalyst tubes are present and each catalyst tube has a length inthe range from 1.5 to 6.0 m and a wall thickness in the range from 2.0to 4.0 mm and an internal diameter in the range from 20 to 90 mm. 32.The process as claimed in claim 29, wherein the deflection plates have athickness in the range from 8 to 30 mm.
 33. The process as claimed inclaim 29, wherein the reactor (reactors) is (are) constructedsymmetrically about a cross-sectional plane in the middle of thereactor.
 34. The process as claimed in claim 29, wherein the reactor(the reactors) has (have) a plurality of zones, with the individualzones being separated from one another in a liquid-tight manner bydividing plates so that the heat transfer medium cannot flow from onezone into the other within the reactor (the reactors).
 35. The processas claimed in claim 29, wherein a sheath for accommodating amultithermocouple with two or more temperature measuring points, whichopens out below the reactor, is provided in at least one of the catalysttubes.
 36. The process as claimed in claim 29, which is conducted in twoor more reactors (1) connected in series, with a concentration measuringpoint for the residual chlorine content and/or a temperature measuringpoint being provided in the connecting piece between the lower cap ofthe upper reactor and the upper cap of the lower reactor.
 37. Theprocess as claimed in claim 36, which is conducted in said one or morecylindrical reactors connected in series, wherein the second reactor hascatalyst tubes having a greater internal tube diameter compared to thefirst reactor, the reactors are connected directly in series withoutcaps located in between and spacers are provided between the reactors.38. The process as claimed in any of claim 29, wherein the reactor (thereactors) is (are) enclosed by a safety chamber.
 39. The process asclaimed in claim 36, wherein the second and/or further reactors has/havea smaller external dimension than the upstream reactor.
 40. The processas claimed in claim 29, wherein water, aqueous sodium hydroxide solutionor one or more chlorinated hydrocarbons is used as heat transfer mediumand the gaseous reaction mixture and the liquid heat transfer medium arepassed through the reactor in cross-countercurrent or incross-concurrent.
 41. The process as claimed in claim 29, wherein carbonmonoxide and chlorine are passed from above or from below through thecatalyst tubes in a molar ratio in the range from 1.01 to 1.10 and thelongitudinal axis of the reactor is vertical.
 42. The process as claimedin claim 29, wherein the solid catalyst installed in the catalyst tubesis activated carbon or an at least partially open-pored carbon foam. 43.The process as claimed in claim 29, wherein the catalyst tubes have aheat transfer coefficient on the heat transfer medium side in the rangefrom 500 to 2000 W/m²/K.